The invention relates to a charging method for storage batteries, in particular for NiCd (nickel-cadmium) and NiH (nickel hydride) cells. In addition, a circuit array for implementation of the method in accordance with the invention is provided.
A large number of portable electronic devices (e.g. portable phones, laptops, camcorders etc.) have become available in recent years, which as a rule however cannot be connected to the power mains using a lead. Accordingly, battery-powered operation must be ensured, with these batteries having to be kept in a continuous state of operational readiness. The problem this entails is that the battery must be charged as fully as possible, but without damaging it by overcharging.
The various charging methods are constant current charge mode, delta temperature/delta time charge mode, negative delta voltage charge mode, positive delta voltage charge mode, pulse charge mode and reflex charge mode.
The first two methods are the least expensive ones. In the first-named method, the battery is continuously overcharged with a low current (0.1 I). The expenditure for a constant current source is low, however the long charging time is a drawback and rapidly leads to damage of the cells. It is also standard practice to restrict the charging time in this charging method. The charging operation is therefore broken off as soon as a defined time has elapsed. However, this does not take into account the charge state, for which reason similar problems are encountered here as in permanent overcharging.
With delta temperature/delta time charging, the charging current is switched off once a certain temperature has been reached, for example 45.degree. C., or once a difference between the ambient temperature and the battery temperature has been reached. This method too ignores the charge state of the battery. Also, it cannot be ruled out that at high ambient temperatures the cells might suffer damage. Finally, the temperature curve of these damaged cells is absolutely unpredictable.
The damage to the cells occurring with the methods described lead to a premature reduction of the available capacity.
In the negative delta voltage charge mode, the fall in the charge curve after complete charging of the battery is used as the switch-off criterion. If batteries, particularly NC and NH batteries, are charged from a constant current source, the charging voltage rises steadily for as long as the cell is capable of converting the supplied energy into chemical energy. When the batteries are no longer capable of storing the supplied energy, the latter is converted into heat and the cell voltage drops, with the end of charging being recognized at the same time. This method can however only be used for those battery types permitting high current charging. This charging method itself has the drawback that during the quick-charge operation surface effects take place in the cell, leading to fluctuations in the battery voltage and causing a premature break-off of the charging operation. For that reason, long-term integration over several measuring cycles, or better still repeated measured value recording and storage, is necessary to detect incorrect measured values by mathematical operations or to rule out such values by interpolation. Realization of a long-term integration by means of analog circuit technology requires considerable expenditure, whereas mathematical processing of the measured values entails the use of a microprocessor. Since NC batteries have a considerably more pronounced charging voltage curve than NH batteries, a more precise evaluation of the charge curve is essential for the latter, for example over several measured values. NH batteries are always overcharged with this charging method.
In the positive delta voltage charge mode, the gradient of the rising charge curve is evaluated. With the battery almost completely charged, the rise in the charging voltage decreases again. By mathematical differentiation of the charge curve, the reduction in the rise can be evaluated as the charge stop criterion. Since several different mathematical operations have to be performed with this method, a microprocessor is almost the only possible solution, as is proposed in U.S. Pat. No. 4,746,852.
The pulse charging mode is known from EP-A-03 11 460. Charging is achieved with very high current pulses. The battery voltage is measured here during a currentless phase and compared with a fixed reference value. The charging operation is ended as soon as the battery voltage has reached this reference value. Since the batteries have manufacturer-related differences in their voltage situations, this fixed reference value has a negative effect with regard to optimum utilisation of the capacity and protection from overcharging.
Charging using the reflex mode is the most expensive of the methods described so far, since it also entails the use of a microprocessor. By this method, a charge pulse of a certain duration is followed by a short, high-voltage discharge pulse, which is in turn followed by a short currentless phase used for measurement of the charging voltage.
A charging circuit for NiCd batteries in particular is known from DE-OS 30 40 852, in which the drop of the charge curve after complete charging of the batteries is used as the switch-off criterion for the charging operation, i.e. the negative delta voltage charge mode already described above is applied. This known circuit contains a digital memory whose stored value is adjusted to the charging voltage at preset intervals during charging, until the time at which the charging voltage reaches its maximum value. When the charging operation is continued, the charging voltage drops below the stored maximum value. A comparison of the actual charging voltage of the battery with the stored maximum value supplies the criterion for ending the charging operation.
The digital memory in this circuit is made up of a staircase voltage generator and an oscillator, with a binary counter serving as the staircase voltage generator. A drawback of this known circuit is that the charging voltage measured during the charging operation leads to poor charging results, since this voltage value also contains those resistance components based on the specific resistance, the electrode resistance and the electrolyte resistance, and not only on the electrochemical cell potential, which in itself best reproduces the charge state of a battery. A further drawback can be discerned in that the charging operation is ended too soon, i.e. before the battery is fully charged, in the event that the charging voltage drops for a short time and then rises again. This known circuit does not however ensure that the batteries are charged in every case with maximum capacity.